This software is still in development, so we don’t recommend you run it on a production site. Consider setting up a test site just to play with the new version. To test WordPress 4.9, try the WordPress Beta Tester plugin (you’ll want “bleeding edge nightlies”). Or you can download the beta here (zip).

For more information on what’s new in 4.9, check out the Beta 1 blog post. Since the Beta 1 release, we’ve made 70 changes in Beta 2, and 92 changes in Beta 3. In Beta 4, we’ve made 80 changes, focusing on bug fixes and finalizing new features.